1. Field of the Invention
The present invention relates to a high-frequency circuit device such as a wave guide or a resonator, having two parallel planar conductors, and a communication apparatus employing such a high-frequency circuit device.
2. Description of the Related Art
A variety of transmission lines may be employed in apparatuses operating in the micro-wave band and the millimeter-wave band. The following transmission lines are typically available: (i) a grounded coplanar line composed of a dielectric plate with one side generally coated with a ground electrode and the other side having a coplanar line thereon; (ii) a grounded slot line composed of a dielectric plate with one side coated with a ground electrode and the other side having a slot; and (iii) a planar dielectric line composed of a dielectric plate with both sides having slots.
Each of the above transmission lines usually have two parallel planar conductors. When an electromagnetic field is disturbed by input and output sections and bend sections of the transmission line, a spurious mode wave (also simply referred to as a "spurious mode"), such as a parallel-plate mode wave, is induced and travels between the two parallel planar conductors. For this reason, the leaky spurious mode waves interfere with each other between adjacent lines, presenting the problem of leakage signals.
FIG. 38 illustrates the main transmission mode of a grounded coplanar line and the distribution of a parallel-plate mode electromagnetic field which is generated along with it. As shown, the underside of a dielectric plate 20 is generally coated with an electrode 21 and the top surface of the dielectric plate 20 has a strip conductor 19 and an electrode 22. The electrodes 21 and 22 serve as ground electrodes, and the grounded coplanar line is thus composed of electrodes 21 and 22, the dielectric plate 20 and the strip conductor 19. In such a grounded coplanar line, the electromagnetic field may be disturbed at its edges such that an electric field is established in a direction perpendicular to the electrodes 21 and 22, and a parallel-plate mode electromagnetic field occurs as shown. Solid lines with arrow heads represent the electric field, broken lines represent the magnetic field, and two-dot chain lines represent the distribution of currents.
To control the propagation of such an unwanted mode wave, through holes are conventionally provided along both sides of a transmission line at a pitch shorter than the wavelength of a transmission mode wave, thereby connecting top and bottom electrodes arranged on the top and bottom faces of a dielectric plate.
The through holes, arranged along the direction of propagation for connecting the top and bottom electrodes, serves as a wall (hereinafter referred to as a "electric barrier"), blocking the propagation of the parallel-plate mode wave. However, in a high frequency region, such as the millimeter-wave band, the dielectric plate must be thin to control the generation of harmonic mode waves, and the intervals between the through holes must be extremely short. This involves high processing accuracy in the manufacture of the circuit device.
When no through holes are arranged in the dielectric plate, the dielectric plate having electrodes thereon are entirely housed in a cutoff wave guide. In such a case, however, the dimensions of the cutoff wave guide must be equal to or smaller than half the guide wavelength, and the dimensional requirements of the wave guide become severer.
A portion of the electrode where the spurious mode wave leaks can be partially cut away to form a wall (hereinafter referred to as a "magnetic wall") to block the propagation of the spurious mode wave. This arrangement poses a new problem because the cutout portion of the electrode functions somewhat as a resonator.